1. Field of the Invention
This invention relates generally to touch screen overlays for use on the viewing surface of visual display devices such as computer monitors and, more specifically, this invention relates to a laminate structure for such overlays with greatly improved durability and signal strength.
2. Description of the Prior Art
Touch screen overlays have been used as interactive input devices at the display surface of computer monitors such as cathode ray tubes, gas panel displays, light-emitting diode arrays and other such types of visual display devices. Such overlays have obvious advantages over conventional data input devices such as keyboards by enabling the user to interface more directly with the computer by interreacting directly with the monitor display. Such interaction may be by finger touch or by use of a stylus. The latter is preferable since it enables the user to input data in various forms, including handwritten alphanumeric text and graphics.
One of the first interactive display devices utilized the light pen which has a photodetector near its tip. The light pen is manipulated by placing its tip in proximate contact with a computer monitor screen at a desired location. As the raster scans the face of the monitor, the light dot it produces is detected by the light pen, thus enabling the computer to correlate the position of the pen with coordinates of the raster. This type of interactive display device is not truly a touch sensitive one since it relies essentially on photodetection rather than touch. Light beam arrays interrupted by the presence of a finger or other intrusive object likewise rely principally upon photodetection rather than touch.
One form of touch sensitive display uses a flexible, deformable membrane formed of a transparent laminate which is placed over the display surface of the computer monitor. The laminate generally comprises two conductor planes deposited on a flexible medium so that when the user mechanically displaces one of the conductor planes by a finger or stylus, the conductors are brought into electrical contact with the conductors in the second plane. The electrical resistance of the conductor plane is changed as a function of the position of the touch on the membrane. Appropriate electronics are provided to translate the resistance value into the position corresponding with the touch.
One version of such touch screen membranes is manufactured by the John Fluke Mfg. Co., Inc. and is typified in a number of patents assigned to this company, including U.S. Pat. No. 4,423,299 (Gurol et al) and U.S. Pat. No. 4,696,860 (D. L. Epperson). The touch screen display disclosed in the '299 patent uses a flexible membrane to support a first set of parallel transparent conductors. These conductors face a second set of parallel transparent conductors mounted on a backplate which is secured to the surface of the computer monitor. The second set of conductors faces upward away from the monitor while the first set faces downward toward the monitor. The conductors of one set run horizontally while those of the other set run vertically so when superimposed they form a grid. The different layers are closely spaced apart and held together on the outer periphery by, for example, spring clips. Between the two sets of a conductors is an air gap. Along the outside edge of each layer is a bus which interconnects the conductors supported on that layer. In this manner, electrical signals from the conductors are transmitted to appropriate electronic equipment. When pressure from a finger or stylus is applied to the flexible membrane, the first set of conductors will be deflected downward across the air gap into contact with the second set of conductors mounted on the backplate along the surface of the monitor. Contact between these sets of conductors acts as a mechanical closure of a switch to complete an electrical circuit which is detected by the electronic equipment through the respective buses at the edge of the panel, thus providing a means for detecting the location of the switch closure. To prevent inadvertent contact between the sets of conductors, both the '299 and '860 patents disclose uniformly distributing transparent beads or bumps of non-conductive material to keep the sets of conductors apart in the air gap except when pressed by the operator.
In the mechanically deformable type of membrane described in the aforenoted patents, the first set of conductors in the top membrane layer must flex or bend in order to make physical contact with the the second set of conductors below. The degree of such bending is reduced in a another type of membrane touch screen such as described in U.S. Pat. No. 4,686,332 (Greanias et al), which is assigned to the IBM Corporation. The structure of the touch screen described in this patent is fairly similar to that of the mechanically deformable screens described above, except that the air gap has been replaced with a very thin non-conductive adhesive layer. There is therefor no mechanical switch action such as described above. Rather, the conductors radiate electromagnetic signals out from the surface of the screen. These signals can be detected by a radiative pickup stylus. The presence of a finger can be detected by a change in the capacitance of the conductor array while the location of the stylus is determined by the signal strength of the electro-magnetic radiation emitted by individual ones of the conductors.
The structure of the overlay membrane disclosed in the '332 patent can be better understood with reference to FIGS. 8 and 12 of that patent. The air gap has been replaced by the non-conductive adhesive serving as an insulation layer 52. The overlay 20 in FIG. 12 comprises two major portions, an inner laminate 56 and an outer laminate 58 which are attached by an adhesive layer 52'. The inner laminate is applied over the outer surface of the glass face of a computer monitor. The inner laminate also has an anti-newton ring coating 53 which is applied to the display side of the overlay to eliminate newton rings when the inner laminate comes into contact with the glass face of the monitor and an electrostatic shield layer of transparent conductive material which is grounded and serves to shield the conductors of the overlay from noise generated by the monitor. The inner laminate also contains inner substrate layer 50 which is an optically clear layer of polyethylene terephthalate onto which is sputtered transparent wire coatings of indium tin oxide arranged as parallel vertical conductors facing out from the monitor display. The outer substrate layer 54 of the outer laminate 58 is substantially the same as the inner substrate 50. Indium tin oxide conductors are likewise deposited on the outer substrate layer 54, but facing inward toward the monitor display and oriented at right angles to the vertical conductors deposited on the inner substrate 50. The inner and outer substrates are disposed so that the respective sets of conductors formed thereon face each other. Both the inner and outer substrates are coated on the face containing the conductors with a thin insulation layer of ultraviolet initiated vinyl acrylic polymer, 52 and 52", respectively, and both are joined with adhesive layer 52' of the same composition as the insulation layers. When the inner and outer laminates are joined, layers 52, 52', and 52" become indistinguishable and can be regarded structurally as a single thin insulating layer between the facing sets of transparent conductors.
Touch screen overlays such as disclosed in the foregoing prior art patents are deficient in several important respects. One is the durability of the laminate, particularly the transparent conductors. Indium-tin-oxide (ITO) is typically used for the conductor wires because of its transparent and conductive properties. This compound is a ceramic material belonging to a class of the materials known as transparent conducting oxides. Being ceramic, these materials are quite brittle and can readily crack under tensile forces. Cracking of conductor wires in touch screens is very serious since electrical conductivity and therefor signal transmission will be lost. In the touch screen overlays of the above described prior art, the transparent wire conductors formed on the outer membrane substrate and facing down toward the monitor screen undergo significant tensile forces when a stylus or other hard instrument is forced against the touch screen. This is particularly severe in the prior art touch screens which have an air gap between the two sets of conductors since a large deflection of the top flexible membrane creates large tensile forces in the transparent conductors formed on the inside surface of the membrane. The stylus forces the top flexible membrane downward so that contact is made between the upper and lower conductors. This creates large tensile forces on the inner surface of the upper membrane which carry the first set of wire conductors. These tensile forces can cause the conductor wires to crack and lose their ability to conduct electricity. The touch screen will therefore fail in any area where such cracking has occurred.
Cracking of the conductors due to such tensile forces is also a serious problem in touch screens such as disclosed in the aforenoted U.S. Pat. No. 4,686,332 where the air gap is replaced with the adhesive layer. While this relatively hard, thin layer makes the touch screen more durable, experimental tests with a moving stylus has shown that cracking of the upper ITO conductors still occurs. In fact, a stylus readily cracks the transparent ITO conductors with only hand pressure. The tensile forces are so great at the centerline of the stylus that elongation of the ITO conductors is sufficient to cause considerable cracking problems and, therefor, failure of the touch screen.
While increasing the thickness of the outer substrate layer 54 in the touch screen overlay of the '332 patent would improve conductor durability somewhat, this additional thickness would weaken conductor signal strength significantly. The additional thickness would so attenuate the electromagnetic signals emitted by the the transparent conductors that a change in capacitance due to finger touch could no longer be detected effectively. The strength of the signal received by the stylus would also be weakened significantly.
Moreover, irrespective of the thickness of the upper subtrate, there are significant signal losses in the touch screen overlay of the '332 patent due to the screening of the signal from the lower conductors by the upper conductors. This is due to the close proximity of the two sets of conductors in this prior art overlay, with the two sets of conductors facing one another and separated only by the adhesive layer. The electromagnetic signals emanating from the lower conductors are therefor intercepted and absorbed by the upper conductors, thereby greatly attenuating the signal strength available for detection at the surface of the touch screen overlay.
Another disadvantage of the laminate touch screen disclosed in the aforenoted U.S. Pat. No. 4,686,332 is due to deficiencies of the adhesive layer 52, which is a thin layer of ultraviolet light initiated vinyl acrylic polymer. This adhesive layer has been found to have poor peel strength, due in part to the incompatibility of this adhesive with the base film supporting the transparent wires. The peel strength is also poor due to the inadequate cure of this adhesive because the ultraviolet light needed to cure or initiate the acrylic polymer is readily adsorbed in the other substrates supporting the ITO conductors. Another deficiency is that the adhesive layer, while transparent to light, does not readily lend itself to producing a flat, distortion free touch screen overlay.